Maxillary expansion in the mixed dentition: rapid or semi-rapid?

The purpose of this study was to investigate the effects of rapid maxillary expansion (RME) and semi-rapid maxillary expansion (SRME) in the mixed dentition period. The SRME group consisted of 18 patients (11 girls and 7 boys) with a mean age of 8.63 +/- 1.09 years and the RME group 17 patients (11 girls and 6 boys) with a mean age of 8.78 +/- 1.21 years. A splint type tooth- and tissue-borne modified bonded RME appliance was used, with the patients activating the screw two-quarter turns per day for the first week, followed by one-quarter turn every other day in the SRME group and two-quarter turns per day throughout treatment in the RME group. The average treatment time was 57.16 +/- 21.52 and 21.23 +/- 8.36 days for the SRME and RME groups, respectively. A Wilcoxon signed rank test was used to evaluate the treatment effects [pre-(T(0)) - post-(T(1)) treatment changes] for both the SRME and RME groups and a Mann-Whitney U-test to determine the differences between the two groups (T(0)-T(1) changes SRME versus T(0)-T(1) changes RME). For both groups, the maxillary base, nasal cavity width and upper intercanine and intermolar distances were increased, and the upper molars tipped buccally. The only statistically significant (P < 0.05) difference between two groups was in inferior movement of posterior nasal spine (PNS) relative to the SN plane (SN upper left and right quadrantsPNS). This measurement increased in both groups yet significantly more in the RME group. The results suggest that RME and SRME have similar effects on dentofacial structures both in the transverse, vertical, and sagittal planes.